The invention relates to defining the tension of a continuous material web, and especially to eliminating error components in the definition of the tension of a continuous material web.
Many industrial production or further-processing machines measure the tension of a continuous material web. Examples of such material web materials are papers and other commodities made of fibre, fabrics, threads, plastic and metal films and other materials whose tension is defined during their production or further processing. In connection with paper-making and further processing, for instance, the tension of the paper web is measured and adjusted at several points of the web. These points include coating stations and reel-ups. Measuring the tension of the web and making the adjustments based thereon aims at a uniform quality of the web material and an improved runnability of the production or further-processing machine.
One known manner of defining the tension of a material web is to measure it with one or more force sensors attached to a measuring roll. The force sensor can be mounted on one or more positions mechanically connected to the roll, typically under bearing cups at the ends of the roll, in which case the force sensors at least partly support the measuring roll. When moving to this roll, the material web forms a force component that is in the measuring direction of the sensor. A measuring roll often also has its own rotating drive. The roll can then be used not only to measure but also to transport the material web.
The angle of the material web passing over the measuring roll is often such that the torque of the drive rotating the measuring roll affects the measurement, that is, forms in the measuring sensor in its measuring direction a force component that sums with the actual measurable quantity, that is, with the force generated by the material web to the measuring roll.
This problem has been eliminated earlier by attempting to mount the measuring roll in such a manner that the force component generated by the torque of the drive is as small as possible in the measuring direction of the force sensor. However, the travel of the web does not always permit the mounting of the measuring roll and its sensors in an optimum manner for the measurement.
Another way of alleviating the problem is to adjust the measuring roll drive in such a manner that the measuring roll does not pull or slow down the material web. However, this limits the control of the web, because the speed difference of the measuring roll drive and the force directed to the web through it cannot be freely set.
A problem with the above motor drive-equipped measuring arrangement is thus an error component formed in the measurement when the measuring roll is used to actively control the web. However, when the measuring roll is driven at the speed of the web so that it does not pull or slow down the web, it is not actively utilised in controlling the web.